95-702 OCT Masters of Information Systems Management 1 Organizational Communications and Distributed Object Technologies Lecture 3: Models and Architectures95-702 OCT Masters of Information Systems Management 2 System Models • Architectural Models Placement of parts and the relationships between them Examples: client-server model peer process model • Fundamental Models Formal description of properties that are common in the architectural models focusing on the dependability characteristics (correctness, reliability, and security)95-702 OCT Masters of Information Systems Management 3 Main Architectural Models of DS • Client processes • Server processes • Peer processes – cooperate and communicate in a symmetrical manner to perform a task95-702 OCT Masters of Information Systems Management 4 Software and hardware service layers in distributed systems Applications, servicesComputer and network hardwarePlatformOperating system MiddlewareMask heterogeneity and provides a convenient programming model to applications Services such as communications, data sharing, naming,security, transactions, persistent storage and event notification95-702 OCT Masters of Information Systems Management 5 Client-Server Model ServerClientClientinvocationresultServerinvocationresultProcess:Key:Computer:95-702 OCT Masters of Information Systems Management 6 A service provided by multiple servers ServerServerServerServiceClientClientReplication is used to increase performance and availability and to improve fault tolerance95-702 OCT Masters of Information Systems Management 7 Proxy server and caches ClientProxyWeb serverWeb serverserverClientA cache is a store of recently used data objects that is closer than the objects themselves. Caches may be collocated with each client or may be located in a proxy server that can be shared by several clients. This approach may increase availability and performance.95-702 OCT Masters of Information Systems Management 8 A distributed application based on peer processes CoordinationApplicationcodeCoordinationApplicationcodeCoordinationApplicationcodeNo server All processes play a similar role Consider a distributed white board where each application contacts the middle ware layer to perform event notification95-702 OCT Masters of Information Systems Management 9 Napster: peer-to-peer file sharing with a centralized, replicated index Napster serverIndex1. File location2. List of peersrequestoffering the filepeers3. File request4. File delivered5. Index update Napster serverIndex95-702 OCT Masters of Information Systems Management 10 Variations on client-server (mobile code) a) client request results in the downloading of applet code Web serverClientWeb serverAppletApplet codeClientb) client interacts with the applet95-702 OCT Masters of Information Systems Management 11 Thin Clients Thin!Client!Application!Process!Compute server!network!The client does graphics only. Think “Old Text terminal” with a GUI. Applications run on the sever. Examples include: Unix X-11 display protocol Citrix and Microsoft put Microsoft apps in a mainframe model95-702 OCT Masters of Information Systems Management 12 Network Computer NC!File server!network!The OS and Applications are downloaded to the client. No disk (or very little) is included. A disk would act as a cache. Oracle and Sun were once behind this approach. Remember the Javastation?95-702 OCT Masters of Information Systems Management 13 Mobile Agents • Includes both code and data. • Copies itself from one machine to another. • Collects information and returns. • The Xerox PARC worm carried out useful work on idle processors. 1982 • Big security concerns but doable. • The disk with the Robert Morris Internet worm is displayed at the Boston Museum of Science (under glass!)95-702 OCT Masters of Information Systems Management 14 Mobile devices and Spontaneous networking Internet!gateway!PDA!service!Music !service! service Discovery!Alarm!Camera!Guests!devices!Laptop!TV/PC !Hotel wireless!network!95-702 OCT Masters of Information Systems Management 15 Fundamental Models All of the above systems share some fundamental properties: Interaction Model communication takes place with delays and without a universal clock Failure Model correct operation of DS is threatened by various types of faults Security Model the security model defines and classifies forms of attack95-702 OCT Masters of Information Systems Management 16 Interaction Model • Two Variants: Synchronous Distributed System The model assumes we can place bounds on time needed for process execution, communication, and clock drift. Asynchronous Distributed Systems assumes we can set no bounds on any of the above (some design problems can be solved even with these assumptions) The internet fits this model well.95-702 OCT Masters of Information Systems Management 17 Lamport Clocks (1) Three processes, each with its own clock. P0 P1 P2 0 6 12 18 24 30 36 42 48 54 60 0 8 16 24 32 40 48 56 64 72 80 0 10 20 30 40 50 60 70 80 90 100 A B C D95-702 OCT Masters of Information Systems Management 18 Lamport Clocks (2) Lamport defined the “happens-before” relation. The expression a->b is read “a happens before b”. If a and b are two events within the same process and a occurs before b then a -> b is true. If a is the sending of a message by one process and b is the reception of that message by another then a -> b is true. “Happens-before” is transitive. If a->b and b->c then a->c.95-702 OCT Masters of Information Systems Management 19 Lamport Clocks (3) We need a way of assigning time values so that all processes (P) agree that: If a->b then C(a) < C(b) with C always increasing never decreasing.95-702 OCT Masters of Information Systems Management 20 Lamport Clocks (4) Lamport’s Algorithm LC1: C(i) is incremented before each event happens at P(i) LC2: (a) When a process P(i) send a message m, it piggybacks on m the value t = C(i) (b) On receiving (m,t), a process P(j) computes C(j) = max(C(j), t) and then applies
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